Internationalised Computer Science Exercises - Correction

Richard Wordingham via Unicode unicode at
Thu Feb 1 17:45:07 CST 2018

On Thu, 1 Feb 2018 01:38:58 +0000
Richard Wordingham via Unicode <unicode at> wrote:

> I believe the concurrent star of a language A is (|A|)*, where
> |A| = {x ∊ A : {x}* is a regular language}
> (The definition works for the trace of fully decomposed Unicode
> character strings under canonical equivalence.)

I misremembered.  The notation is (/A/)*, where starter-free x ∊ A is
dealt with by converting x into its maximal substrings all of whose
characters are of the same canonical combining class and putting them
in /A/ in place of x.

> Concurrent star is not a perfect generalisation.  If ab = ba, then
> X = {aa, ab, b} has the annoying property that X* is a regular trace
> language, but |X|* is a proper subset of X*.  For Unicode, X would be
> a rather unusual regular language.

So this is the other way round.  We will get /X/ = {aa, a, b}, so X* is
a proper subset of /X/*.


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